The availability of low cost and small microelectromechanical systems (MEMS) accelerometers has created a large variety of new commercial applications with their inclusion in smart phones and other handheld or mobile devices. As compared with other types of MEMS accelerometer technologies, a thermal MEMS accelerometer, which is based on the principle of measuring internal changes of convection heat transfer due to the acceleration applied, has superior advantages for consumer applications, including a low cost fabrication process, high reliability and very good shock or impact resistance. A thermal MEMS three-axis accelerometer usually employs a 2D (2-dimension) sensing structure to achieve three-axis acceleration measurements. As is known, X and Y acceleration measurements can be achieved accurately by their differential signal pickoff mechanizations. Recent advances in thermal MEMS accelerometer design and manufacture further improve bias (zero offset) stability of X and Y axes measurements over time and temperature.
The stability of Z-axis bias, however, is still a real challenge in the industry. Generally, the Z-axis signal, which is picked off the structure by observing the temperature differences at different times of X and Y axes sensing, is a very weak and secondary signal in the three-axis acceleration sensing operation. As a result, the Z-axis measurements have a significantly larger bias drift over temperature as compared to those in the X and Y axes.
In order to provide an accurate three-axis accelerometer at a price point that meets the “low cost philosophy,” which is critical for consumer applications, it is highly desirable to achieve a low cost approach for calibrating all axes measurements, especially the Z-axis, of a thermal MEMS three-axis accelerometer.